The present invention relates to carrier particles of a two-component dry-type developer for developing latent electrostatic images to visible images for use in electrophotography, electrostatic recording methods and electrostatic printing methods. More particularly, it relates to carrier particles coated with a silicone resin layer containing an organic tin compound serving as a catalyst for hardening the silicone resin layer in the course of the formation of the silicone resin layer on a core material, with the concentration of the organic tin compound being changed in the direction of the thickness of the layer towards the outer surface of the carrier particles.
Conventionally, a variety of two-component dry-type developers are known, which comprise carrier particles and toner particles. In such two-component dry-type developers, toner particles which are usually much smaller than the carrier particles are triboelectrically attracted to the carrier particles and are held on the surface of the carrier particles. The electric attraction between the toner particles and the carrier particles is caused by the friction between the toner particles and the carrier particles. When the toner particles held on the carrier particles are brought near or into contact with a latent electrostatic image, the electric field of the latent electrostatic image works on the toner particles to separate the toner particles from the carrier particles, overcoming the bonding force between the toner particles and the carrier particles. As a result, the toner particles are attracted towards the latent electrostatic image, so that the latent electrostatic image is developed to a visible toner image. In the two-component dry-type developers, only the toner particles are consumed as development is performed. Therefore, it is necessary to replenish the toner particles from time to time in the course of the development of latent electrostatic images.
Further, it is necessary that the carrier particles be capable of triboelectrically charging toner particles to the desired polarity and with a sufficient amount of charges, and that the amount of the electric charges and the polarity thereof be maintained for a long period of time for use. In a conventional two-component type developer, it is apt to occur that the surface of the carrier particles is eventually covered with a resin released from the toner particles in the course of the mechanical mixing of the toner particles with the carrier particles in the development apparatus. Once the phenomenon of the surface of the carrier particles being covered with such resin takes place, which is generally referred to as the "spent phenomenon", such carrier particles no longer function as active carrier particles, that is, carrier particles which contribute to development. As a result, the charging characteristics of the carrier particles in the conventional two-component type developer deteriorate with time while in use. In the end, it is necessary to replace the entire developer by a new developer.
In order to prevent the spent phenomenon, a method of coating the surface of carrier particles with a variety of resins has been proposed. However, resins which can in fact prevent the spent phenomenon have not been discovered. At one extreme, for instance, carrier particles coated with a styrene methacrylate copolymer or polystyrene are excellent in the triboelectric charging properties. However, since the surface energy of the carrier particles is comparatively high, the carrier particles are easily covered with the resin released from the toner particles while in use. In other words, the spent phenomenon occurs easily and, accordingly, the life of such developer is short.
The above-mentioned "spent phenomenon" scarcely takes place in the carriers coated with polytetrafluoroethylene polymers, since such carriers have a low surface energy. However, since the polytetrafluoroethylene polymers are positioned on the extreme negative side in the triboelectric series, the carriers coated with the polytetrafluoroethylene polymers cannot be employed for charging toners to a negative polarity.
As carrier particles coated with a coating layer having a low surface energy, for example, the following carrier particles have been proposed, which are coated with a layer containing a silicone resin having a low surface energy:
Carrier particles coated with a coating layer in which an unsaturated silicone resin, an organosilicon, silanol and other resins are mixed with a styrene-acrylic resin, as disclosed in U.S. Pat. No. 3,562,533; PA1 Carrier particles coated with a coating layer comprising a polyphenylene resin and an organosilicon-containing terpolymer resin, as disclosed in U.S. Pat. No. 3,847,127; PA1 Carrier particles coated with a coating layer comprising a styrene-acrylate-methacrylate resin, organosilane, silanol, siloxane and other resins, as disclosed in U.S. Pat. No. 3,627,522; PA1 Carrier particles coated with a coating layer comprising a silicone resin and a nitrogen-containing resin having positive charging characteristics, as disclosed in Japanese Laid-Open Patent Application No. 55-127567; and PA1 Carrier particles coated with a coating layer comprising a modified silicone resin, as disclosed in Japanese Laid-Open Patent Application No. 55-157751.
The Q/M (the quantity of electric charges per unit weight of developer) of some dry-type developers for developing latent electrostatic images comprising silicone resin coated carriers and toner particles increases while in use, and the Q/M of other dry-type developers decreases as the thickness of the silicone resin coated layer of the carrier particles decreases while in use, with the result that the charging capability of the carrier particles changes while in use. Such changes in the Q/M also depend upon the kinds of employed toners and the polarity to which the toner particles are charged. However, the reasons why the Q/M of some developers decreases and the Q/M of other developers increases while in use are unknown. In any event, if the Q/M of the developer changes while in use, the developing performance changes and therefore good toner images cannot be obtained in a stable manner.